Method and arrangement for positioning a sensor head for measuring while object is moving

ABSTRACT

During measurement by means of a measurement arrangement ( 4 ) in which a sensor head ( 7 ) is axially movable in a sensor housing ( 6 ) and is intended, in the measurement position, to rest against a measured object ( 2 ) via an air cushion, the orientation of the sensor head in the sensor housing is monitored. When the orientation of the sensor housing deviates by a predetermined value from the normal, the measurement arrangement is temporarily moved a distance away from the measured object and is then returned to the measurement position agains for continued measurement, with the sensor head now in the normal position relative to the sensor housing. In such a measurement arrangement, a monitoring device ( 24 ) is provided to automatically produce the stated movement of the measurement arrangement.

[0001] The present invention relates to a method for measurement,according to the pre-characterizing clause of patent claim 1, and to anarrangement for measurement, according to the pre-characterizing clauseof patent claim 5.

PRIOR ART

[0002] In many situations involving the measurement of, for example,material thicknesses, a type of measurement equipment is used in which asensor head is movably mounted in a sensor housing and in which thesensor head is held at a certain distance from a measured object bymeans of air which is blown out between the sensor head and the measuredobject and there forms an air cushion. By means of such an air cushion,the gap between sensor head and measured object can be kept small andconstant, which is of advantage for measurement precision.

[0003] However, as regards measurement precision, it is also importantthat the sensor head can move easily in the sensor housing, which isnormally mounted in a fixed manner. By means of an air-type bearing, itis possible for the sensor head to be made easily movable in the axialdirection in the sensor housing, so that it can adapt its position tothe actual thickness of the measured object.

[0004] During control measurement of the thickness of a material webrunning across a roller, it is desirable to be able to move themeasurement equipment backward and forward along the roller in order toeffect continuous measurement. Any irregularities on the material webcan in this case result in the sensor head being set at an inclinationand no longer being able to move entirely freely in the axial direction.The measurement result may consequently be incorrect, possibly withoutthis being noticed. In addition, as a result of being locked in aninclined position, the sensor head may be subjected to considerable wearand tear against the material web. Another problem is that of being ableto rectify the error before too much damage has occurred on account ofomitted measurement control and sensor wear.

OBJECT OF THE INVENTION

[0005] The aim of the invention is to make it possible to avoid theabove-mentioned problems and to achieve improved reliability andincreased measurement precision. Another aim is to do this by simplemeans.

DISCLOSURE OF THE INVENTION

[0006] The object of the invention is achieved on the one hand by meansof a method having the features set out in patent claim 1, and on theother hand by means of a measurement arrangement having the features setout in patent claim 5.

[0007] By detecting the position of inclination of the sensor headrelative to the sensor housing and—if there is an indication ofexcessive inclination and resulting difficulty of movement of the sensorhead—by initiating a displacement and a subsequent repositioning of themeasurement arrangement, it is possible in a simple way to restore fullmobility to the sensor head. A control arrangement in the measurementarrangement effects continuous monitoring to ensure that the sensor headhas the necessary mobility in the sensor housing, and it canautomatically warn of an error and correct the error. The extraequipment which is needed in the measurement arrangement can be madesimple and reliable.

[0008] Further features and advantages of the invention are set out inthe following description and in the patent claims.

[0009] The invention is described in more detail below on the basis ofan illustrative embodiment which is shown on the attached drawing.

DESCRIPTION OF THE FIGURES

[0010] In the drawing:

[0011]FIG. 1 shows a schematic view of a measurement station with ameasurement arrangement according to the invention,

[0012]FIG. 2 shows a partial cross section through a measurementarrangement according to the invention,

[0013]FIG. 3 shows a circuit diagram of a measurement arrangementaccording to the invention, and

[0014]FIG. 4 shows a diagram of the output signal from a monitoringdevice.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0015] At a measurement station 1 shown in FIG. 1, a material web 2, forexample in the form of a newly rolled sheet of metal, is guided across aroller 3. To measure the thickness of the material web 3, a measurementarrangement 4 according to the invention is mounted so as to movebackward and forward along the roller 3 in a frame 5.

[0016] As can be seen more clearly from FIG. 2, the measurementarrangement 4 includes a sensor housing 6 and a sensor head 7 which isarranged movably in the latter, a sensor 8 being connected in aconventional manner via an arrangement of wires 9 to a measurementapparatus (not shown here). The sensor head 7 is provided with a tubularshaft 10 which runs through a control part 12 which is fixed in a bore11 in the sensor housing 6 and which is sealed tight. Mounted on top ofthe shaft 10 is a body 13 in which control elements 14 are secured.These control elements 14 have their free ends in recesses 15 in thecontrol part 12 and in this way prevent mutual turning between thesensor head 7 and the control part 12. Compressed air is fed into thetop of the sensor housing and can flow through the inside of the shaft10 and out through an opening 16 at the bottom of the sensor housing 7in order to form an air cushion against the material web 2, as isindicated by arrows.

[0017] The shaft 10 on the sensor head 7 can move easily in the controlpart 12 by virtue of air-type bearing. For this purpose, compressed airis conveyed via an attachment 17 in the sensor housing 6 and is fed viachannels 18 in the control part 12 to the central hole in the controlpart 12 in which the shaft 10 runs with a certain play. Mounted at anaxial distance from each other in the control part 12 are two annularcontact elements 19, 20 which serve as a guide for the shaft 10 andwhich are connected each to a line 21, 22. Between the two contactelements 19, 20 in the control part 12 there is an annular dampingmember 23 which surrounds the shaft 10 and acts on the flow of air alongthe outside thereof.

[0018] As can be seen more clearly from FIG. 3, the two contact elements19, 20 form, together with a number of other components, a monitoringdevice 24 included in the measurement arrangement 4. The contactelements 19, 20 are connected in parallel via lines 21 and 22,respectively, and their respective resistors R1 and R2, to the input ofan alarm mechanism 25 which in turn is connected to a maneuvering device26. The input of the alarm mechanism 25 is also connected to earth via acapacitor C. The shaft 10 which is electrically conductive at least onthe outside is connected via a line 27 to the positive pole of a voltagesource.

[0019] The shaft 10 normally runs free from the two contact elements 19,20 in the control part 12 and can therefore easily change its heightposition relative to the material web 2. When the sensor head 7 isexposed to a lateral force, for example as a result of an irregularityin the material web 2, the shaft 10 can be inclined so far that itsability to move is limited or ceases. It may then come into contact withthe lower contact element 20, or possibly the upper contact element 19,if the inclination is limited, or in contact with both contact elements19, 20 if the inclination is greater. As a result of such contact,current flows to the alarm 25, the current strength I being a functionof how strong the inclination is. An example of what can happen is shownin FIG. 4 where a curve D shows how the current strength I can vary as afunction of the time t. The curve portion a represents contact only withthe lower contact element 20, while the curve portion b representscontact only with the upper contact element 19, and the curve portion crepresents contact with both contact elements 19, 20, i.e. a strongdegree of inclination. The horizontal lines E, F and G indicatedifferent alarm levels for the degree of inclination.

[0020] At the lower level E, when the shaft 10 is not in contact withany of the contact elements 19, 20, everything functions as normal. Atthe intermediate level F, an inclination has taken place which is ofsuch a degree that an alarm signal is emitted in order to indicate thatthe situation is abnormal, for example by triggering of a warning light.At the uppermost level G, the inclination has become so great that thefunction of the equipment is jeopardized, for which reason themaneuvering device 26 is activated to move the whole measurementarrangement 4 away from the material web 2 and thereby remove the causeof the inclination. Thereafter, the measurement arrangement 4 isreturned automatically to the measurement position against the materialweb 2, and the sensor head 7 resumes its normal measuring positionagainst the material web 2, without inclination of the shaft 10 and withthe alarm signal deactivated. Such a movement is shown diagrammaticallyby the double arrow 28 in FIG. 1.

[0021] By not immediately moving the measurement arrangement 4 away inthe event of a small inclination, it is possible to allow a minimaldisturbance on the material web to pass without taking any steps. Onlywhen a greater disturbance occurs is the measurement arrangementautomatically moved away in order to prevent damage and wear of theequipment. It is of course possible to select levels for different stepsin some other way than that indicated here, depending on whichconditions and requirements apply.

[0022] In the example chosen here, the resistance at the resistor R2 isexpediently twice as great as the resistance at the resistor R1, forexample 200 ohm and 100 ohm respectively, but other values can also bechosen, and the resistance at the resistor R2 can also be less than theresistance at the resistor R1, depending on the aims and requirements.

[0023] With the diameter of the shaft 10 being up to ca. 8.00 mm, theinternal diameter of the contact elements 19, 20 is expediently up toca. 8.15 mm, giving a relatively small gap between shaft and contactelement in order to achieve a high degree of precision in the shaftmovement. Additionally providing a good fit between the shaft 10 and thedamping member 23 ensures that the air introduced into the control part12 has an advantageous flow around the shaft 10, so that the sensor head7 is held in a stable and substantially vibration-free measurementposition. At the stated diameter of ca. 8.00 mm of the shaft 10, theinternal diameter of the damping member 23 can expediently be up to ca.8.03 mm, but other values can of course be chosen in order to give othervalues for the gap size and thereby achieve a desired behavior of thesensor head 7.

[0024] To make it easier to mount the contact elements 19, 20 and thedamping member 23 in the control element 12, said control element canexpediently be designed in parts, preferably made of electricallynonconductive material. The contact elements 19, 20 and the shaft 10 aremade of electrically conductive material, at least on those surfaceswhich are facing toward each other and are intended to be able to comeinto contact with each other. The sensor head 7 is normally orientedvertically, but, by suitable design of the measurement arrangement 4,other orientations are also possible. The stated movement of themeasurement arrangement in the event of a considerable inclination ofthe sensor head in such cases naturally takes place in the normaldirection of movement of the measurement head. The sensor 8 included inthe measurement arrangement 4 can preferably work in accordance with thereluctance principle, but it is of course possible to use other types ofsensors.

1. A method for measurement by means of a measurement arrangement (4) inwhich a sensor head (7) is axially movable in a sensor housing (6) andis intended, in the measurement position, to rest against a measuredobject (2) via an air cushion, characterized in that the inclination ofthe sensor head (7) in the sensor housing (6) is monitored, and in that,when the inclination of the sensor head in the sensor housing reaches apredetermined value the measurement arrangement (4) is temporarily moveda distance away from the measured object and is thereafter returned tothe measurement position relative to the measured object for continuedmeasurement, with the sensor head in the normal measuring positionrelative to the sensor housing.
 2. The method as claimed in claim 1,characterized in that the inclination of the sensor head (7) isrepresented by an electrical signal generated upon contact betweensensor head and sensor housing.
 3. The method as claimed in claim 2,characterized in that the electrical signal is generated using twocontact elements (19, 20) against the sensor housing which are separatedfrom each other in the axial direction of the sensor housing.
 4. Ameasurement arrangement in which a sensor head (7) is axially movable ina sensor housing (6) and projects axially out from the latter in orderto rest against a measured object (2) via an air cushion, the sensorhousing being designed to supply compressed air for holding the sensorhead in the sensor housing and for forming the air cushion,characterized in that the measurement arrangement includes a monitoringdevice (24) for monitoring the inclination of the sensor head (7)relative to the sensor housing (6), in that the monitoring device isconfigured in such a way that, when the inclination of the sensor headrelative to the sensor housing reaches a predetermined value duringmeasurement, it activates a maneuvering device (26) for temporarilymoving the measurement arrangement a distance away from the measuredobject and thereafter returning the measurement arrangement to themeasurement position relative to the measured object for continuedmeasurement, with the sensor head in the normal measuring positionrelative to the sensor housing.
 5. The measurement arrangement asclaimed in claim 4, characterized in that the monitoring device (24)includes at least two contact elements (19, 20) mounted at a distancefrom each other in the sensor housing (6) and cooperating with theoutside of a portion (10) of the sensor head (7) projecting into thesensor housing.
 6. The measurement arrangement as claimed in claim 5,characterized in that the contact elements (19, 20) are annular, forcooperation with a cylindrical shaft (10) on the sensor housing (7). 7.The measurement arrangement as claimed in claim 6, characterized in thatthe contact elements (19, 20) are connected electrically to an alarmmechanism (25) which, at a predetermined signal level resulting fromcontact between the contact elements (19, 20) and the shaft (10) on thesensor head (7), is designed to send a maneuver signal to themaneuvering device (26).
 8. The measurement arrangement as claimed inclaim 7, characterized in that the contact elements (19, 20) areconnected in parallel to the alarm mechanism (25).
 9. The measurementarrangement as claimed in claim 8, characterized in that the contactelements (19, 20) are connected each via its resistor (R1, R2), of whichone preferably has a greater resistance than the other.
 10. Themeasurement arrangement as claimed in any of claims 5 to 9,characterized in that the contact elements (19, 20) are mounted axiallyseparate from each other in a control part (12) in which the sensor headis mounted and via which air for supporting the sensor head (7) issupplied.